Muscles can only pull, so a joint needs a pair of muscles to move in two directions. The muscle that contracts makes the movement, and its partner relaxes.
This lesson follows relating bones, joints and muscles in the support and movement chapter.
What happens at the elbow?
| Movement | Biceps | Triceps | Result |
|---|---|---|---|
| Bending the arm (flexion) | Contracts | Relaxes | Forearm lifts toward the shoulder |
| Straightening the arm (extension) | Relaxes | Contracts | Forearm moves away from the shoulder |
The tendons transfer the pull to the forearm bones. The elbow is a hinge joint, so the forearm swings around it like a lever.
Worked example: explain lowering a cup
Question: a student lifts a cup to drink, then lowers it to the table. Explain the action of the muscles in both movements. [4 marks]
A full answer:
- To lift the cup, the biceps contracts and the triceps relaxes.
- The contracting biceps pulls on the forearm through a tendon, so the arm bends at the elbow.
- To lower the cup, the triceps contracts and the biceps relaxes.
- The contracting triceps pulls on the forearm, which straightens the arm, and the biceps is stretched.
Each step names the muscle, the action and the result. Use the phrase “a pair of antagonistic muscles” and add that the two work in opposite ways.
The mistake that costs marks
The common slip is to write that the triceps “pushes” the arm straight. A muscle cannot push, so the answer must say the triceps contracts and pulls.
| Wrong | Right | |
|---|---|---|
| Straightening | Triceps pushes the forearm | Triceps contracts and pulls the forearm straight |
| Bending | Biceps and triceps both contract | Biceps contracts while triceps relaxes |
| Bone to bone | A tendon joins two bones | A ligament joins two bones |
If both muscles contracted together, the forearm would not move, and the joint would be held stiff.
Check yourself
A gymnast holds an arm straight out to the side and does not move it. Explain how the muscles of the upper arm keep the elbow straight.
Answer
The triceps contracts to keep the arm straight, and the biceps also contracts slightly to hold the joint steady. The two pull against each other with balanced force.
The arm stays still because the pulls of the antagonistic pair are balanced.
What to study next
Next, compare how different organisms are supported in comparing support in selected organisms. The biological data interpretation trainer provides more diagrams to read.
Log reversed muscle pairs in the mistake log and paper-error review. For a teacher to check your explanations, see online one-to-one Biology tuition.